74LS93 Mod-5 Problem

[skeatz]

I've noticed a strange behavior when simulating a mod-5 counter using the 74LS93 or 74LS293 - instead of counting from 0 to 4, the counter displays 2 - 4 consistently. Other modulo numbers, e.g. mod-4 or mod-6 count correctly.

The problem disappears when I add a gate to the counter outputs, before connecting to the reset inputs.

I've attached a file with both circuits to illustrate the problem.

Does anyone know what is the cause of this problem? I suspect it is due to a timing issue. Is there a problem with the SPICE model for the component?

thanks,

steven

[pegru001]

13 years later...

One of my students just discovered the same thing. File is attached (this is for PLTW DE 3.2.3 proc 1).

The oscilloscope clearly shows channel B "clearing" to 1.

Being able to restart the count only at 0 is supposed to be a disadvantage of the 74LS93N.